The present invention relates generally to a carbon fiber composition and method for making same. Specifically, the present invention relates to a carbon fiber composition having a controlled, predetermined electrical resistance level and method for making same.
Composite materials comprising laminated plies of carbon fiber reinforced in a resin matrix are often used due to their high strength to weight ratio. However, in addition to the structural aspects, many applications can greatly benefit from the composite materials having controlled electrical resistance levels. Such applications including engine components, aircraft or land based weapon systems, and heating elements of various kinds, including resistance heating elements, thermal heaters and flexible heating units.
Currently, to make use of carbon fibers in a composite having controlled, lower resistance levels, the amount of carbon fibers must be evenly distributed at extremely low concentrations, or loading levels, such as less than 0.1 percent carbon by weight, due to the relatively high conductivity of the carbon fibers. It is extremely difficult to work with carbon fibers at this low concentration level. Alternately, carbon fibers have been incorporated into paper-thin plies that are sandwiched between layers of highly resistive structural composites. Although this construction method has produced promising results, the process of arranging the plies is manual, and hence, is extremely expensive, and impractical for commercial use.
Another alternate construction method is commingling the carbon fibers with nonconductive fibers. However, due to the high conductivity of carbon fibers, the ratio of nonconductive fibers, such as glass or quartz filaments, to each carbon segment would need to be several thousand to one. Further, due to the relatively high degree of brittleness, the resulting material simply cannot be handled without breakage. Currently, techniques of blending glass and carbon exist, such as stretch-breaking, but even stretch-breaking can only blend glass and carbon fibers within a ratio range of about several hundred to one.
In conventional polyacrylonitrile (PAN) carbon fiber processing, the electrical conductivity of the resulting carbon fibers is affected by the carbonization temperature, the carbonization time and the internal orientation of the carbon planes within a filament. For example, referring to FIG. 1 for prior art carbon fibers, the y axis corresponds to the log of resistivity values which likewise correspond to the carbonization temperature (x-axis) to produce a nonlinear relationship between resistivity and temperature. As is shown, the log of resistivity values ranges from about 3,100 micro-ohms per cm at about 1,000° C. to less than 1,000 micro-ohms per cm at about 2,750° C. This range of resistivity values is not acceptable in many electrically sensitive applications.
Carbon fibers having an increased level of electrical resistance are commercially available. However, these carbon fibers are produced by attempting to carbonize the carbon fibers at a lower temperature than is used to produce carbon fibers having lower resistance levels. In fact, at these lower temperatures, the carbon fibers are not fully carbonized, that is, they are not fully converted to carbon, and are believed to retain nitrogen impurities in the fibers. The carbon fibers produced at the lower carbonizing temperatures by this method are not environmentally stable in that their electrical resistance levels change significantly over time, even at room temperature, and further vary with a change in temperature. The reason for the fluctuation in resistance of the carbon fibers is believed to be caused by the retained nitrogen which results in an increased propensity of the carbon fibers to accumulate moisture in an unpredictable manner.
Therefore, what is needed is a method for integrating carbon fibers into a composite construction having a wide range of controllable electrical resistance levels that remains substantially unchanged over a wide range of operating conditions which is easy and inexpensive to make.